The present invention relates to a method and an apparatus for purifying a raw material gas for use in manufacturing a semiconductor device, and also to a purifier used in said purifying method and a method of manufacturing said purifier.
In the process for manufacturing semiconductor devices, there has been known to form amorphous or mono- or poly-crystalline film of semiconductor material such as Si, Ge, P and As by using a metal hydride gas such as monosilane (SiH.sub.4), disilane (Si.sub.2 H.sub.6), germanium hydride (GeH.sub.4), phosphorus hydride (PH.sub.3) and arsenic hydride (AsH.sub.3).
For instance, an amorphous hydrogenated silicon which is a kind of amorphous semiconductor material has been used as a solar cell, a photo-sensor, a thin-film transistor, etc., and the field of application therefor has gradually widened. Since the maximum light absorption spectrum of amorphous hydrogenated silicon is situated at a shorter wavelength as compared with crystal silicon, the hydrogenated amorphous silicon is preferably used as a photoelectric transducer such as a solar cell and photosensor, and the hydrogenated amorphous silicon has been used on a commercial basis. However, a drawback of such a transducer is that its characteristics become deteriorated during usage. For example, if strong light is made incident upon the hydrogenated amorphous silicon it becomes highly resistive. Therefore, the photoelectric converting efficiency of the transducer comprising the hydrogenated amorphous silicon is decreased. This effect has been well known as the Steabler-Wronski effect whose mechanism remains almost unknown, and thus various studies about this effect have been carried out from the points of view of a manufacturing method, a growth condition, an included impurity, etc. of the hydrogenated amorphous silicon.
Recently, it has been found that the oxygen contained in the hydrogenated amorphous silicon film is a large contributer to the Steabler-Wronski effect. Usually, hydrogenated amorphous silicon is produced by a glow discharge decomposition method using a monosilane gas (SiH.sub.4). In the hydrogenated amorphous silicon film produced in this manner, oxygen is present in about a 200 ppm atomic ratio (1.times.10.sup.19 atoms/cc Si). In the manufacturing process, oxygen is absorbed in the film during the glow discharge, and a source of the oxygen is regarded as O.sub.2 and H.sub.2 O in the monosilane gas (SiH.sub.4) and O.sub.2, H.sub.2 O remain in the reaction vessel system. Actually, in commercially available monosilane gas, oxygen sometimes comprises about 1.about.10 ppm, and if use is made of the hydride gas mentioned above as the raw mateial gas, oxygen is included in the hydrogenated amorphous silicon film over 200 ppm. Therefore, in order to decrease the oxygen amount in the hydrogenated amorphous silicon film to an amount smaller than 1 ppm, it is necessary to decrease the amount of oxygen contained in the monosilane gas below the order of 0.1 ppm.
Generally, in order to remove a small amount of oxygen contained in the raw material gas, use is made of a metal-carrier catalyst. The metal-carrier catalyst is effective in the case where the raw material gas to be purified is not decomposed in the presence of a catalyst like the deoxygenation from a hydrogen gas, a nitrogen gas or an argon gas. However, this metal-carrier catalyst is not so effective for the deoxygenation in the monosilane gas, because the catalyst becomes inactive due to the monosilane gas. The monosilane gas is easily decomposed into silicon and hydrogen on the metal-carrier catalyst and the decomposed silicon is segregated on the catalyst carrier and functions to deteriorate the hydrogen absorption efficiency. Therefore, such metal-carrier catalyst cannot be used effectively for the deoxygenation of the raw material gas for use in manufacturing semiconductor devices such as monosilane, disilane, germanium hydride gas, phosphorus hydride gas, and arsenic hydride gas.